Foamed sheet

ABSTRACT

The foamed sheet includes a foamed body having an average cell diameter of 10 to 200 μm, and a compression set at 80° C. of not more than 80% and an impact absorption change rate of not more than ±20% defined by: 
       Impact absorption change rate (%)={(an impact absorption rate  b  after high-temperature compression−an initial impact absorption rate  a )/the initial impact absorption rate  a }×100,
 
     where the impact absorption rate a is an impact absorption rate (%) of a test piece A and the impact absorption rate b (%) after high-temperature compression is an impact absorption rate (%) acquired by storing the test piece A at 80° C. for 72 hours in the state of being compressed by 60% with respect to the initial thickness of the test piece A, thereafter releasing the compression state, and thereafter conducting measurement after a lapse of 24 hours at 23° C.

TECHNICAL FIELD

The present invention relates to a foamed sheet which, even if having avery small thickness, is excellent in impact absorption and excellent inheat resistance, and to an electric or electronic device using thefoamed sheet.

BACKGROUND ART

There are conventionally used foamed materials when optical members suchas image display members fixed on image display apparatuses such asliquid crystal displays, electroluminescence displays and plasmadisplays, display members installed on so-called “cellular phones,”“smartphones” and “personal digital assistants,” cameras and lenses arefixed on predetermined sites (for example, housings). As such foamedmaterials, there have been used compression-molded high-densityfine-cell urethanic foamed bodies having a closed cell structure andlow-density urethane and besides, polyethylenic foamed bodies havingclosed cells and an expansion ratio of about 30, and the like.Specifically, there are used, for example, a gasket (see PatentLiterature 1) composed of a polyurethanic foamed body having an apparentdensity of 0.3 to 0.5 g/cm³, and a sealing material for electric orelectronic devices (see Patent Literature 2) composed of a foamedstructural body having an average cell diameter of 1 to 500 μm.

In recent years, along with the thickness reduction of electronicdevices such as PCs (personal computers), tablet PCs, PDAs (personaldigital assistants) and cellular phones, impact absorption sheets havebeen used on panel rear faces for the prevention of breakage of liquidcrystal panels, organic EL panels and the like. Then, the thicknessreduction is demanded also on the impact absorption sheets. In the casewhere conventional foamed materials are used as such impact absorptionsheets, however, the sheets cannot exhibit sufficient impact absorption.

Further along with the function enhancement of electronic devices, inheat generating bodies such as electronic components, the amount of heatgeneration becomes large. When conventional foamed materials are usedfor electric or electronic devices having such heat generating bodiesexhibiting a large amount of heat generation, a problem which arises isthat the performance reduces due to heat accumulated in the interiorsand the electric or electronic devices are broken due to impact whenbeing dropped or otherwise.

CITATION LIST Patent Literature Patent Literature 1: Japanese PatentLaid-Open No. 2001-100216 Patent Literature 2: Japanese Patent Laid-OpenNo. 2002-309198 SUMMARY OF INVENTION Technical Problem

Therefore, an object of the present invention is to provide a foamedsheet which, even if having a very small thickness, exhibits excellentimpact absorption and is excellent in heat resistance in which theperformance does not reduce even under high temperatures.

Another object of the present invention is to provide an electric orelectronic device which, even if being down sized and reduced inthickness, and having a heat generating body exhibiting a large amountof heat generation, is hardly broken due to impact in dropping.

Solution to Problem

As a result of exhaustive studies in order to achieve the above objects,the present inventors have found that: a foamed sheet, comprising afoamed body having a specific average cell diameter and having acompression set at 80° C. of not more than 80% and a small differencebetween the impact absorption after the high-temperature (80° C.)compression and the initial impact absorption, even if the foamed sheethas a very small thickness, exhibits excellent impact absorption and isexcellent in heat resistance as well; and an electric or electronicdevice installed with the foamed sheet, even if having a heat generatingbody exhibiting a large amount of heat generation, is not broken byimpact in dropping. The present invention has been completed by carryingout further studies based on these findings.

That is, the present invention provides a foamed sheet comprising afoamed body having an average cell diameter of 10 to 200 μm, and havinga compression set at 80° C. of not more than 80% and an impactabsorption change rate of not more than ±20% as defined by thefollowing.

Impact absorption change rate (%)={(an impact absorption rate b afterhigh-temperature compression−an initial impact absorption rate a)/theinitial impact absorption rate a}×100

The initial impact absorption rate a: an impact absorption rate (%) of atest piece A

The impact absorption rate b (%) after high-temperature compression: animpact absorption rate (%) acquired by storing the test piece A at 80°C. for 72 hours in the state of being compressed by 60% with respect tothe initial thickness of the test piece A, thereafter releasing thecompression state, and thereafter conducting measurement after a lapseof 24 hours at 23° C.

The impact absorption rate: a value defined by the following expressionin an impact absorption test (the weight of an impactor: 28 g, theswing-up angle: 40°) (23° C.) using a pendulum impact tester.

Impact absorption rate (%)={(F₀−F₁)/F₀}×100 wherein F₀ is an impactforce when the impactor is made to collide with a support plate alone;and F₁ is an impact force when the impactor is made to collide with thesupport plate of a structural body composed of the support plate and thetest piece A.

In the foamed sheet, it is preferable that the thickness is 30 to 1,000μm, and the apparent density of the foamed body is 0.2 to 0.7 g/cm³.

The foamed body preferably has a peak top of a loss tangent (tan δ) inthe range of not less than −30° C. and not more than 30° C., the losstangent (tan δ) being a ratio of a loss elastic modulus to a storageelastic modulus at an angular frequency of 1 rad/sec in a dynamicviscoelasticity measurement.

The foamed body can be formed of at least one resin material selectedfrom the group consisting of acrylic polymers, rubbers, urethanicpolymers and ethylene-vinyl acetate copolymers.

The foamed body may be formed through step A of mechanically foaming anemulsion resin composition. Further the foamed body may be formedfurther through step B of coating a base material with the mechanicallyfoamed emulsion resin composition followed by drying. Step B maycomprise preliminary drying step B1 of drying the bubble-containingemulsion resin composition applied on the base material at not less than50° C. and less than 125° C., and regular drying step B2 of thereafterfurther drying the resultant at not less than 125° C. and not more than200° C.

In the foamed sheet, the compression set at 80° C. is preferably notmore than 50%, and more preferably not more than 25%.

In the foamed sheet, the thickness is preferably 40 to 500 μm, and morepreferably 50 to 300 μm.

The apparent density of the foamed sheet is preferably 0.21 to 0.6g/cm³, and more preferably 0.22 to 0.5 g/cm³.

The foamed sheet may have a pressure-sensitive adhesive layer on oneface or both faces of the foamed body.

The foamed sheet may be one to be used as an impact absorption sheet forelectric or electronic devices.

The present invention also provides an electric or electronic deviceusing the foamed sheet. The electric or electronic device includes onehaving a display member, and having a structure in which the foamedsheet is interposed between a housing of the electric or electronicdevice and the display member.

Advantageous Effects of Invention

The foamed sheet according to the present invention, since it comprisesa foamed body having a specific average cell diameter and has acompression set at 80° C. of not more than 80% and an impact absorptionchange rate of as low as not more than ±20%, even if having a very smallthickness, exhibits excellent impact absorption and is excellent in heatresistance as well. Hence, even in the case where the foamed sheet isused for electric or electronic devices having heat generating bodiesexhibiting a large amount of heat generation, and the like, theperformance as an impact absorption sheet does not reduce and a highreliability can be attained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic constitution view of a pendulum impact tester(impact testing apparatus).

FIG. 2 is a view illustrating a schematic constitution of a holdingmember of the pendulum impact tester (impact testing apparatus).

DESCRIPTION OF EMBODIMENTS

The foamed sheet according to the present invention comprises a foamedbody having an average cell diameter of 10 to 200 μm. The lower limit ofthe average cell diameter of the foamed body is preferably 15 μm, andmore preferably 20 μm; and the upper limit is preferably 150 μm, morepreferably 130 μm, and still more preferably 100 μm. Since the averagecell diameter is not less than 10 μm, the foamed sheet exhibitsexcellent impact absorption. Further since the average cell diameter isnot more than 200 μm, the foamed sheet is excellent in compressionrecovery as well. Here, the maximum cell diameter of the foamed body is,for example, 40 to 800 μm, and the lower limit thereof is preferably 60μm, and more preferably 80 μm; and the upper limit is preferably 400 μm,and more preferably 220 μm. Further the minimum cell diameter of thefoamed body is, for example, 5 to 70 μm, and the lower limit thereof ispreferably 8 μm, and more preferably 10 μm; and the upper limit ispreferably 60 μm, and more preferably 50 μm.

In the foamed sheet according to the present invention, the compressionset at 80° C. is not more than 80%, preferably not more than 50%, morepreferably not more than 25%, and especially preferably not more than10%.

A compression set test at 80° C. can be carried out according to theprovision of JIS K6262. The compression set (%) is determined by thefollowing expression.

CS={(t0−t1)/(t0−t2)}×100

CS: a compression set (%)

t0: an original thickness (mm) of a test piece

t1: a thickness (mm) of the test piece 30 min after the test piece isremoved from a compression apparatus

t2: a thickness (mm) of the test piece in the state of being under acompressive strain

Here in the present invention, the compression set is a value when thetest piece is compressed by 60%.

In the foamed sheet according to the present invention, the impactabsorption change rate defined by the following is not more than ±20%,preferably not more than ±15%, and still more preferably not more than±5%.

Impact absorption change rate (%)={(an impact absorption rate b afterhigh-temperature compression−an initial impact absorption rate a)/theinitial impact absorption rate a}×100

The initial impact absorption rate a: an impact absorption rate (%) of atest piece A

The impact absorption rate b (%) after high-temperature compression: animpact absorption rate (%) acquired by storing the test piece A at 80°C. for 72 hours in the state of being compressed by 60% with respect tothe initial thickness of the test piece A, thereafter releasing thecompression state, and thereafter conducting measurement after a lapseof 24 hours at 23° C.

The impact absorption rate: a value defined by the following expressionin an impact absorption test (the weight of an impactor: 28 g, theswing-up angle: 40°) (23° C.) using a pendulum impact tester.

Impact absorption rate (%)={(F₀−F₁)/F₀}×100 wherein F₀ is an impactforce when the impactor is made to collide with a support plate alone;and F₁ is an impact force when the impactor is made to collide with thesupport plate of a structural body composed of the support plate and thetest piece A.

Since the foamed sheet according to the present invention has a lowcompression set at 80° C., and a low rate of the impact absorptionchange rate, even if the foamed sheet is compressed at a hightemperature, cells hardly collapse and excellent thickness recovery isattained; and even in the case where the foamed sheet is subjected to animpact at a high temperature, high impact absorption is exhibitedsimilarly as at normal temperature. Hence, also in the case where thefoamed sheet is used for electric or electronic devices having heatgenerating bodies exhibiting a large amount of heat generation, and thelike, even if an impact is applied on the electric or electronic devicesin dropping or otherwise, breakage of the devices can be prevented.

The schematic constitution of a pendulum impact tester (impact testingapparatus) will be described by way of FIG. 1 and FIG. 2. As illustratedin FIG. 1 and FIG. 2, an impact testing apparatus 1 (pendulum tester 1)is constituted of a holding member 3 as a holding means to hold a testpiece 2 (foamed sheet 2) by an arbitrary holding force, an impactapplying member 4 to apply an impact stress on the test piece 2, apressure sensor 5 as an impact force detecting means to detect an impactforce on the test piece 2 by the impact applying member 4, and the like.Further the holding member 3 to hold the test piece 2 by an arbitraryholding force is constituted of a fixing jig 11, and a pressing jig 12facing the fixing jig 11 and being slidable so that the test piece 2 isinterposed and held between the fixing jig 11 and the pressing jig 12.Further the pressing jig 12 is provided with a pressure adjusting means16. Further the impact applying member 4 to apply an impact force on thetest piece 2 held by the holding member 3 is constituted of a supportrod 23 (shaft 23) whose one end 22 is rotatably supported on a supportcolumn 20 and whose other end side has an impactor 24, and an arm 21 tolift and hold the impactor 24 at a predetermined angle. Here since asteel ball is used as the impactor 24, by providing an electromagnet 25on one end of the arm, the impactor 24 is enabled to be unifiedly liftedat the predetermined angle. Further the pressure sensor 5 to detect animpact force acting on the test piece 2 by the impact applying member 4is provided on the face side of the fixing jig 11 opposite to the facethereof contacting the test piece.

The impactor 24 is a steel ball (iron ball). Further the angle (swing-upangle a in FIG. 1) at which the impactor 24 is lifted by the arm 21 is40°. The weight of the steel ball (iron ball) is 28 g.

As illustrated in FIG. 2, the test piece 2 (foamed sheet 2) isinterposed between the fixing jig 11 and the pressing jig 12 through asupport plate 28 constituted of a highly elastic plate material such asa resinous plate material (acryl plate, polycarbonate plate or the like)or a metal plate material.

The impact absorption is calculated by the expression described beforeby using the above impact testing apparatus, and determining an impactforce F₀ measured by closely fixing the fixing jig 11 and the supportplate 28 on each other and then making the impactor 24 collide with thesupport plate 28, and an impact force F₁ measured by inserting andclosely fixing the test piece 2 between the fixing jig 11 and thesupport plate 28 and then making the impactor 24 collide with thesupport plate 28. Here, the impact testing apparatus is a similarapparatus as used in Example 1 of Japanese Patent Laid-Open No.2006-47277.

The foamed sheet according to the present invention has excellent impactabsorption while having a small thickness. The impact absorption rate(the weight of the impactor: 28 g, the swing-up angle: 40°) is usually 5to 70%, and the lower limit is preferably 10%, more preferably 20%, andstill more preferably 28%; and the upper limit is preferably 60%.

The thickness of the foamed sheet according to the present invention isnot especially limited, but is, for example, 30 to 1,000 μm, and thelower limit thereof is more preferably 40 μm, and still more preferably50 μm; and the upper limit is more preferably 500 μm, still morepreferably 300 μm, and especially preferably 200 μm. When the thicknessof the foamed sheet is not less than 30 μm, cells can be incorporateduniformly, and better impact absorption can be exhibited. Further bymaking the thickness of the foamed sheet to be not more than 1,000 μm,the foamed sheet can easily conform to fine clearances. The foamed sheetaccording to the present invention, even if having as small a thicknessas 30 to 1,000 μm, is excellent in impact absorption.

In the present invention, from the viewpoint of the impact absorption,the ratio of the average cell diameter (μm) to the thickness (μm) of thefoamed sheet (the former/the latter) is preferably in the range of 0.1to 0.9. The lower limit of the ratio of the average cell diameter (μm)to the thickness (μm) of the foamed sheet is preferably 0.2, and morepreferably 0.3; and the upper limit is preferably 0.85, and morepreferably 0.8.

The apparent density of the foamed body constituting the foamed sheetaccording to the present invention is not especially limited, but ispreferably 0.2 to 0.7 g/cm³. The lower limit thereof is preferably 0.21g/cm³, and more preferably 0.22 g/cm³; and the upper limit is preferably0.6 g/cm³, more preferably 0.5 g/cm³, and especially preferably 0.4g/cm³. When the apparent density of the foamed body is not less than 0.2g/cm³, a high strength can be maintained; and when it is not more than0.7 g/cm³, higher impact absorption is exhibited. Further when theapparent density of the foamed body is in the range of 0.2 to 0.4 g/cm³,much higher impact absorption is exhibited.

In the case where the thickness of the foamed sheet is large in somedegree, the impact absorption can be regulated by selection of theaverage cell diameter, the apparent density and the like; in the casewhere the thickness of the foamed sheet is very small (for example, thethickness is 30 to 500 μm), however, the impact cannot sufficiently beabsorbed only by regulation of these properties, in some cases. This isbecause in the case where the thickness of the foamed sheet is verysmall, cells in the foamed body instantly collapse by an impact and theimpact buffering function by cells disappears. From such a viewpoint, itis preferable that the peak top of the loss tangent (tan δ), which is aratio of a loss elastic modulus to a storage elastic modulus at anangular frequency of 1 rad/sec in a dynamic viscoelasticity measurementof the foamed body, is present in the range of not less than −30° C. andnot more than 30° C. In such a way, even after cells collapse, theconstituting material of the foamed body exhibits more a function ofbuffering impacts.

The lower limit of the temperature range where the peak top of the losstangent is present is more preferably −25° C., still more preferably−20° C., and especially preferably −10° C.; and the upper limit is morepreferably 20° C., and still more preferably 10° C. In the case ofmaterials having not less than two peak tops of the loss tangent, atleast one of the peak tops is desirably in the above range. When thepeak temperature is not less than −30° C., better compression recoveryis exhibited. Further when the peak temperature is not more than 30° C.,higher flexibility is exhibited and better impact absorption isexhibited.

It is preferable that the peak top intensity (maximum value) of the losstangent (tan δ) in the range of not less than −30° C. and not more than30° C. is higher from the viewpoint of the impact absorption, and thepeak top intensity is, for example, not less than 0.2, and preferablynot less than 0.3. The upper limit value of the peak top intensity(maximum value) is, for example, 2.0.

The peak temperature of the loss tangent (tan δ) contributes to theimpact absorption of the foamed body in such a manner, in many cases.Although the reason is not completely clear why when the peak top of theloss tangent (tan δ), which is a ratio of a loss elastic modulus to astorage elastic modulus at an angular frequency of 1 rad/sec in adynamic viscoelasticity measurement of the foamed body, is present inthe range of not less than −30° C. and not more than 30° C., the impactabsorption of the foamed sheet becomes high, it is presumably due tothat the peak of the loss tangent (tan δ) is present in frequenciescorresponding to those of impacts. That is, it is presumed that sincethe range of not less than −30° C. and not more than 30° C. of the losstangent (tan δ) is reduced to a range of frequencies corresponding todropping impacts of a structural material based on the temperature-timeconversion rule in viscoelasticity measurements, foamed sheets having apeak temperature of the loss tangent (tan δ) in the range of not lessthan −30° C. and not more than 30° C. exhibit higher impact absorption.Further the storage elastic modulus is a resilient force to an impactenergy applied on the foamed sheet; and when the storage elastic modulusis high, the impact is repulsed as it is. By contrast, the loss elasticmodulus is a physical property which converts an impact energy appliedon the foamed sheet to heat; since a higher loss elastic modulus causesan impact energy to be converted to more heat, the impact is absorbedand the deformation is reduced. It is presumed from this that foamedsheets which convert impacts to more heat and exhibit a lower resilientforce, that is, exhibit a higher loss tangent (tan δ), which is a ratioof a loss elastic modulus to a storage elastic modulus, exhibit a higherimpact absorption rate.

With respect to the foamed body constituting the foamed sheet accordingto the present invention, the composition, the cell structure and thelike thereof are not especially limited as long as the foamed body hasthe above properties. The cell structure may be any of an open cellstructure, a closed cell structure and a semi-open semi-closed cellstructure. From the viewpoint of the impact absorption, an open cellstructure and a semi-open semi-closed cell structure are preferable.

The foamed body can be constituted of a resin composition containing aresin material (polymer). Here, it is preferable that the peak top ofthe loss tangent (tan δ), which is a ratio of a loss elastic modulus toa storage elastic modulus at an angular frequency of 1 rad/sec in adynamic viscoelasticity measurement of the resin composition in anunfoamed state [the resin composition (solid material) in the case ofnot being foamed], is in the range of not less than −30° C. and not morethan 30° C. The lower limit of the temperature range where the peak topof the loss tangent is present is more preferably −25° C., still morepreferably −20° C., and especially preferably −10° C.; and the upperlimit is more preferably 20° C., and still more preferably 10° C. In thecase of materials having not less than two peak tops of the losstangent, at least one of the peak tops is desirably in the above range.A higher peak top intensity of the loss tangent (tan δ) in the range ofnot less than −30° C. and not more than 30° C. of the resin composition(solid material) is preferable from the viewpoint of the impactabsorption, wherein the value of the peak top intensity corresponds to avalue obtained by dividing a peak top intensity of the loss tangent (tanδ) in the range of not less than −30° C. and less than 30° C. of thefoamed body by an apparent density (g/cm³) of the foamed body. The peaktop intensity of the loss tangent (tan δ) in the range of not less than−30° C. and not more than 30° C. of the resin composition (solidmaterial) is preferably not less than 0.9 (g/cm³)⁻¹, and the upper limitis, for example, about 3 (g/cm³)⁻¹.

A resin material (polymer) constituting the foamed body is notespecially limited, and publicly or commonly known resin materialsconstituting foamed bodies can be used. Examples of the resin materialinclude acrylic polymers, rubbers, urethanic polymers and ethylene-vinylacetate copolymers. Among these, from the viewpoint of the impactabsorption, acrylic polymers, rubbers and urethanic polymers arepreferable. The resin material (polymer) constituting the foamed bodymay be of a single kind or of not less than two kinds.

In order to make the peak top of the loss tangent (tan δ), which is aratio of a loss elastic modulus to a storage elastic modulus at anangular frequency of 1 rad/sec in a dynamic viscoelasticity measurementof the foamed body, to be in the range of not less than −30° C. and notmore than 30° C., Tg of the resin material (polymer) can be made to bean index or an indication. The resin material (polymer) can be selected,for example, from resin materials (polymers) having a Tg in the range ofnot less than −50° C. and less than 50° C. (the lower limit ispreferably −40° C., and more preferably −30° C.; and the upper limit ispreferably 40° C., and more preferably 30° C.)

The acrylic polymer is preferably one formed of, as essential monomercomponents, a monomer having a Tg of its homopolymer of not less than−10° C. and a monomer having a Tg of its homopolymer of less than −10°C. By using such an acrylic polymer and regulating the amount ratios ofthe former monomer and the latter monomer, there can comparativelyeasily be obtained a foamed body having a peak top of the loss tangent(tan δ), which is a ratio of a loss elastic modulus to a storage elasticmodulus at an angular frequency of 1 rad/sec in a dynamicviscoelasticity measurement, of not less than −30° C. and not more than30° C.

Here, a “glass transition temperature (Tg) when a homopolymer is formed”(simply referred to as “Tg of a homopolymer” in some cases) in thepresent invention means a “glass transition temperature (Tg) of ahomopolymer of the corresponding monomer”; and numerical values arespecifically cited in “Polymer Handbook” (3rd edition, John Wiley &Sons, Inc., 1987). Here, Tgs of homopolymers of monomers which are notdescribed in the above literature are values acquired, for example, bythe following measurement method (see Japanese Patent Laid-Open No.2007-51271). That is, 100 parts by weight of a monomer, 0.2 parts byweight of 2,2′-azobisisobutyronitrile, and 200 parts by weight of ethylacetate as a polymerization solvent are charged in a reaction vesselequipped with a thermometer, a stirrer, a nitrogen introducing tube anda refluxing cooling tube, and stirred for 1 hour under the introductionof nitrogen gas. After oxygen in the polymerization system is removed insuch a way, the system is heated up to 63° C. and allowed to react for10 hours. Then, the system is cooled to room temperature to therebyobtain a homopolymer solution having a solid content concentration of33% by weight. Then, the homopolymer solution is cast and applied on aseparator, and dried to thereby fabricate a test sample (sheet-likehomopolymer) having a thickness of about 2 mm. Then, the test sample ispunched out into a disc of 7.9 mm in diameter, and interposed betweenparallel plates; and the viscoelasticity is measured by using aviscoelasticity tester (ARES, manufactured by Rheometric Scientific,Inc.) and in a temperature region of −70 to 150° C. at atemperature-rise rate of 5° C./min in a shearing mode under a shearingstrain of 1 Hz in frequency, and the peak top temperature in tan δ isdefined as Tg of the homopolymer. Here, also Tg of the resin material(polymer) can be measured by this method.

In a monomer having a Tg of its homopolymer of not less than −10° C.,the Tg is, for example, −10° C. to 250° C., preferably 10 to 230° C.,and more preferably 50 to 200° C.

Examples of the monomer having a Tg of its homopolymer of not less than−10° C. include (meth)acrylonitrile; amide group-containing monomerssuch as (meth) acrylamide and N-hydroxyethyl(meth)acrylamide;(meth)acrylic acid; alkyl (meth)acrylates having a Tg of theirhomopolymer of not less than −10° C., such as methyl methacrylate andethyl methacrylate; isobornyl (meth)acrylate; heterocycle-containingvinyl monomers such as N-vinyl-2-pyrrolidone; and hydroxylgroup-containing monomers such as 2-hydroxyethyl methacrylate. These canbe used singly or in combinations of not less than two. Among these,(meth)acrylonitrile (particularly acrylonitrile) is especiallypreferable. When (meth)acrylonitrile (particularly acrylonitrile) isused as a monomer having a Tg of its homopolymer of not less than −10°C., probably because of the strong intermolecular interaction, the peaktop intensity of the loss tangent (tan δ) of the foamed body can be madehigh.

In the monomer having a Tg of its homopolymer of less than −10° C., theTg is, for example, not less than −70° C. and less than −10° C.,preferably −70° C. to −12° C., and more preferably −65° C. to −15° C.

Examples of the monomer having a Tg of its homopolymer of less than −10°C. include alkyl (meth)acrylates having a Tg of their homopolymer ofless than −10° C., such as ethyl acrylate, butyl acrylate and2-ethylhexyl acrylate. These can be used singly or in combinations ofnot less than two. Among these, C₂₋₈ alkyl acrylates are especiallypreferable.

The content of a monomer having a Tg of its homopolymer of not less than−10° C. with respect to the whole monomer components (total amount ofmonomer components) forming the acrylic polymer is, for example, 2 to30% by weight, and the lower limit is preferably 3% by weight, and morepreferably 4% by weight; and the upper limit is preferably 25% byweight, and more preferably 20% by weight. The content of a monomerhaving a Tg of its homopolymer of less than −10° C. with respect to thewhole monomer components (total amount of monomer components) formingthe acrylic polymer is, for example, 70 to 98% by weight, and the lowerlimit is preferably 75% by weight, and more preferably 80% by weight;and the upper limit is preferably 97% by weight, and more preferably 96%by weight.

Here, when the monomer forming the acrylic polymer contains a nitrogenatom-containing copolymerizable monomer, and when an emulsion resincomposition is subjected to shearing by a mechanical stirring or thelike to be thereby caused to foam, the viscosity of the compositiondecreases and it becomes easy for a large number of bubbles to beentrapped in the emulsion; and thereafter, when the emulsion resincomposition containing bubbles is applied on a base material and driedin its standing-still state, since the composition becomes easilyaggregated and the viscosity increases, and the bubbles are held in thecomposition and it becomes difficult for the bubbles to diffuse outside,a foamed body excellent in the foamed property can be obtained.

Examples of the nitrogen atom-containing copolymerizable monomer(nitrogen atom-containing monomer) include cyano group-containingmonomers such as (meth)acrylonitrile; lactam ring-containing monomerssuch as N-vinyl-2-pyrrolidone; and amide group-containing monomers suchas (meth)acrylamide, N-hydroxyethyl(meth)acrylamide,N-methylolacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide anddiacetoneacrylamide. Among these, preferable are cyano group-containingmonomers such as acrylonitrile, and lactam ring-containing monomers suchas N-vinyl-2-pyrrolidone. The nitrogen atom-containing monomers can beused singly or in combinations of not less than two.

In an acrylic polymer having a structural unit originated from such anitrogen atom-containing monomer, the content of the structural unitoriginated from the nitrogen atom-containing monomer is, with respect tothe whole structural units constituting the acrylic polymer, preferably2 to 30% by weight, and the lower limit thereof is more preferably 3% byweight, and still more preferably 4% by weight; and the upper limitthereof is more preferably 25% by weight, and still more preferably 20%by weight.

Further in an acrylic polymer having a structural unit originated fromsuch a nitrogen atom-containing monomer, in addition to the structuralunit originated from the nitrogen atom-containing monomer, a structuralunit originated from a C₂₋₁₈ alkyl acrylate (particularly a C₂₋₈ alkylacrylate) is preferably contained. The C₂₋₁₈ alkyl acrylates can be usedsingly or in combinations of not less than two. In such an acrylicpolymer, the content of the structural unit originated from a C₂₋₁₈alkyl acrylate (particularly a C₂₋₈ alkyl acrylate) is, with respect tothe whole structural units constituting the acrylic polymer, preferably70 to 98% by weight, and the lower limit thereof is more preferably 75%by weight, and still more preferably 80% by weight; and the upper limitthereof is more preferably 97% by weight, and still more preferably 96%by weight.

The rubber may be either of natural rubber and synthetic rubber.Examples of the rubber include nitrile rubber (NBR), methylmethacrylate-butadiene rubber (MBR), styrene-butadiene rubber (SBR),acrylic rubber (ACM, ANM), urethane rubber (AU) and silicone rubber.Among these, preferable are nitrile rubber (NBR), methylmethacrylate-butadiene rubber (MBR) and silicone rubber.

Examples of the urethanic polymer include polycarbonate-basedpolyurethane, polyester-based polyurethane and polyether-basedpolyurethane.

As the ethylene-vinyl acetate copolymer, publicly or commonly knownethylene-vinyl acetate copolymers can be used.

The foamed body constituting the foamed sheet, in addition to the resinmaterial (polymer), may contain, as required, a surfactant, acrosslinking agent, a thickener, a rust preventive, a silicone-basedcompound and other additives.

For example, for the micronization of the cell diameter and thestabilization of cells foamed, an optional surfactant may be contained.The surfactant is not especially limited, and there may be used any ofan anionic surfactant, a cationic surfactant, a nonionic surfactant, anamphoteric surfactant and the like; but from the viewpoint of themicronization of the cell diameter and the stabilization of cellsfoamed, an anionic surfactant is preferable, and a fatty acidammonium-based surfactant, particularly ammonium stearate or the like,is more preferable. The surfactants may be used singly or incombinations of not less than two. Further dissimilar surfactants may beused concurrently, and for example, an anionic surfactant and a nonionicsurfactant, or an anionic surfactant and an amphoteric surfactant may beused concurrently.

The amount [solid content (nonvolatile content)] of the surfactant to beadded is, with respect to 100 parts by weight of the resin material(polymer) [solid content (nonvolatile content)], for example, 0 to 10parts by weight, and the lower limit is preferably 0.5 parts by weight;and the upper limit is preferably 8 parts by weight.

Further in order to improve the strength, heat resistance and moistureresistance of the foamed body, an optional crosslinking agent may becontained. The crosslinking agent is not especially limited, and eitherof an oil-soluble one and a water-soluble one may be used. Examples ofthe crosslinking agent include epoxy-based, oxazoline-based,isocyanate-based, carbodiimide-based, melamine-based and metaloxide-based ones. Among these, oxazoline-based crosslinking agents arepreferable.

The amount [solid content (nonvolatile content)] of the crosslinkingagent to be added is, with respect to 100 parts by weight of the resinmaterial (polymer) [solid content (nonvolatile content)], for example, 0to 10 parts by weight, and the lower limit is preferably 0.01 parts byweight, and more preferably 0.1 parts by weight; and the upper limit ispreferably 9 parts by weight, and more preferably 8 parts by weight.

Further for the stabilization of cells foamed and the improvement of thefilm formability, an optional thickener may be contained. The thickeneris not especially limited, and includes acrylic acid-based, urethanicand polyvinyl alcoholic ones. Among these, polyacrylic acid-basedthickeners and urethanic thickeners are preferable.

The amount [solid content (nonvolatile content)] of the thickener to beadded is, with respect to 100 parts by weight of the resin material(polymer) [solid content (nonvolatile content)], for example, 0 to 10parts by weight, and the lower limit is preferably 0.1 parts by weight;and the upper limit is preferably 5 parts by weight.

Further in order to prevent the corrosion of metal members adjacent tothe foamed sheet, an optional rust preventive may be contained. The rustpreventive is preferably an azole ring-containing compound. When anazole ring-containing compound is used, both the corrosion prevention ofmetals and the close adhesion with objects can be met simultaneously inhigh levels.

The azole ring-containing compound suffices as long as being a compoundhaving 5-membered ring containing not less than one nitrogen atom in thering, and examples include compounds having a diazole (imidazole,pyrazole) ring, a triazole ring, a tetrazole ring, an oxazole ring, anisoxazole ring, a thiazole ring or an isothiazole ring. These rings maybe condensed with an aromatic ring such as a benzene ring to therebyform condensed rings. Examples of compounds having such a condensed ringinclude compounds having a benzimidazole ring, a benzopyrazole ring, abenzotriazol ring, a benzoxazole ring, a benzisoxazole ring, abenzothiazole ring or a benzisothiazole ring.

The azole ring and the condensed rings (benzotriazole ring,benzothiazole ring and the like) may each have a substituent. Examplesof the substituent include alkyl groups having 1 to 6 carbon atoms(preferably having 1 to 3 carbon atoms) such as a methyl group, an ethylgroup, a propyl group, an isopropyl group and a butyl group; alkoxygroups having 1 to 12 carbon atoms (preferably having 1 to 3 carbonatoms) such as a methoxy group, an ethoxy group, an isopropyloxy groupand a butoxy group; aryl groups having 6 to 10 carbon atoms such as aphenyl group, a tolyl group and a naphthyl group; an amino group; (mono-or di-) C₁₋₁₀ alkylamino groups such as a methylamino group and adimethylamino group; amino-C₁₋₆ alkyl groups such as an aminomethylgroup and 2-aminoethyl group; mono- or di-(C₁₋₁₀ alkyl)amino-C₁₋₆ alkylgroups such as an N,N-diethylaminomethyl group and anN,N-bis(2-ethylhexyl)aminomethyl group; a mercapto group; alkoxycarbonylgroups having 1 to 6 carbon atoms such as a methoxycarbonyl group and anethoxycarbonyl group; a carboxyl group; carboxy-C₁₋₆ alkyl groups suchas a carboxymethyl group; carboxy-C₁₋₆ alkylthio groups such as2-carboxyethylthio group; N,N-bis(hydroxy-C₁₋₄ alkyl)amino-C₁₋₄ alkylgroups such as an N,N-bis(hydroxymethyl)aminomethyl group; and a sulfogroup. Further the azole ring-containing compound may form a salt suchas a sodium salt or a potassium salt.

From the viewpoint of the rust preventive action on metals, preferableare compounds in which an azole ring forms a condensed ring with anaromatic ring such as a benzene ring; and among these, especiallypreferable are benzotriazole-based compounds (compounds having abenzotriazole ring) and benzothiazole-based compounds (compounds havinga benzothiazole ring).

Examples of the benzotriazole-based compounds include1,2,3-benzotriazole, methylbenzotriazole, carboxybenzotriazole,carboxymethylbenzotriazole,1-[N,N-bis(2-ethylhexyl)aminomethyl]benzotriazole,1-[N,N-bis(2-ethylhexyl)aminomethyl]methylbenzotriazole,2,2′-[[(methyl-1H-benzotriazol-1-yl)methyl]imino]bisethanol, and sodiumsalts thereof.

Examples of the benzothiazole-based compounds include2-mercaptobenzothiazole, 3-(2-(benzothiazolyl)thio)propionic acid, andsodium salts thereof.

The azole ring-containing compounds may be used singly or incombinations of not less than two.

The amount [solid content (nonvolatile content)] of the rust preventive(for example, the azole ring-containing compound) [solid content(nonvolatile content)] to be added suffices as long as being in therange of not impairing the close adhesion with objects and the intrinsicproperty of the foamed body, and is, for example, with respect to 100parts by weight of the resin material (polymer) [solid content(nonvolatile content)], for example, preferably 0.2 to 5 parts byweight. The lower limit thereof is more preferably 0.3 parts by weight,and still more preferably 0.4 parts by weight; and the upper limitthereof is more preferably 3 parts by weight, and still more preferably2 parts by weight.

Further in order to improve the recovery and the recovery speed of thethickness of the foamed sheet after being compressed, a silicone-basedcompound may be added. Further for the same purpose, a silicone-modifiedpolymer (for example, a silicone-modified acrylic polymer, asilicone-modified urethanic polymer) may be used as at least a part ofthe resin material (polymer). These can be used singly or incombinations of not less than two.

As the silicone-based compound, preferable are silicone-based compoundshaving not more than 2,000 siloxane bonds. Examples of thesilicone-based compounds include silicone oils, modified silicone oilsand silicone resins.

Examples of the silicone oils (straight silicone oils) include dimethylsilicone oils and methyl phenyl silicone oils.

Examples of the modified silicone oils include polyether-modifiedsilicone oils (polyether-modified dimethyl silicone oils and the like),alkyl-modified silicone oils (alkyl-modified dimethyl silicone oils andthe like), aralkyl-modified silicone oils (aralkyl-modified dimethylsilicone oils and the like), higher fatty acid ester-modified siliconeoils (higher fatty acid ester-modified dimethyl silicone oils and thelike) and fluoroalkyl-modified silicone oils (fluoroalkyl-modifieddimethyl silicone oils and the like).

Among these, polyether-modified silicones are preferable. Examples ofcommercially available products of the polyether-modified silicone oilsinclude straight chain-type ones such as “PEG 11 Methyl EtherDimethicone,” “PEG/PPG-20/22 Butyl Ether Dimethicone,” “PEG-9 MethylEther Dimethicone,” “PEG-32 Methyl Ether Dimethicone,” “PEG-9Dimethicone,” “PEG-3 Dimethicone” and “PEG-10 Dimethicone”; and branchedchain-type ones such as “PEG-9 Polydimethylsiloxyethyl Dimethicone” and“Lauryl PEG-9 Polydimethylsiloxyethyl Dimethicone” (which are allmanufactured by Shin-Etsu Silicone).

The silicone resins include straight silicone resins and modifiedsilicone resins. Examples of the straight silicone resins include methylsilicone resins and methyl phenyl silicone resins. Examples of themodified silicone resins include alkyd-modified silicone resins,epoxy-modified silicone resins, acryl-modified silicone resins andpolyester-modified silicone resins.

The total content of the silicone-based compound and the silicone chainmoiety present in the silicone-modified polymer in the foamed body is,with respect to 100 parts by weight of the resin material (polymer) inthe foamed body, for example, 0.01 to 5 parts by weight in terms ofnonvolatile content (in terms of solid content). The lower limit of thetotal content is preferably 0.05 parts by weight, and more preferably0.1 parts by weight; and the upper limit is preferably 4 parts byweight, and more preferably 3 parts by weight. In the case where thetotal content of the silicone component and the silicone chain moiety inthe foamed body is in the above range, the recovery and the recoveryspeed after compression can be improved without impairing the propertiesas the foamed body.

Further the total content of the silicone-based compound and thesilicone chain moiety present in the silicone-modified polymer in thefoamed body is, for example, 0.01 to 5% by weight in terms ofnonvolatile content (in terms of solid content). The lower limit of thetotal content is preferably 0.05% by weight, and more preferably 0.1% byweight; and the upper limit is preferably 4% by weight, and morepreferably 3% by weight. In the case where the total content of thesilicone component and the silicone chain moiety in the foamed body isin the above range, the recovery and the recovery speed aftercompression can be improved without impairing the properties as thefoamed body.

The foamed body constituting the foamed sheet may contain optional othersuitable components in the range of not impairing the impact absorption.Such other components may be contained in one kind thereof alone, or maybe contained in not less than two kinds thereof. Examples of the othercomponents include polymer components other than the above, softeningagents, antioxidants, antiaging agents, gelling agents, curing agents,plasticizers, filling agents, reinforcing agents, foaming agents (sodiumbicarbonate and the like), microcapsules (thermally expandablemicroballs and the like), flame retardants, light stabilizers,ultraviolet absorbents, coloring agents (pigments, dyes and the like),pH regulators, solvents (organic solvents), thermopolymerizationinitiators and photopolymerization initiators. The amounts [solidcontents (nonvolatile contents)] of these components to be addedsuffices as long as being in the range of not impairing the closeadhesion with objects and the intrinsic property of the foamed body, andare each, for example, with respect to 100 parts by weight of the resinmaterial (polymer) [solid content (nonvolatile content)], preferably inthe range of, for example, 0.2 to 60 parts by weight. The amount [solidcontent (nonvolatile content)] of the foaming agent (sodium bicarbonateor the like) to be added is, with respect to 100 parts by weight of theresin material (polymer) [solid content (nonvolatile content)], morepreferably 0.5 to 20 parts by weight. The amount [solid content(nonvolatile content)] of the microcapsule (thermally expandablemicroball or the like) to be added is, with respect to 100 parts byweight of the resin material (polymer) [solid content (nonvolatilecontent)], more preferably 0.2 to 10 parts by weight. The amount [solidcontent (nonvolatile content)] of the filling agent to be added is, withrespect to 100 parts by weight of the resin material (polymer) [solidcontent (nonvolatile content)], more preferably 0.3 to 50 parts byweight.

Examples of the filling agents include silica, clay (mica, talc,smectite and the like), alumina, aluminum hydroxide, hydroxides ofalkaline earth metals (magnesium hydroxide and the like), carbonatesalts of alkaline earth metals (calcium carbonate and the like),titania, zinc oxide, tin oxide, zeolite, graphite, carbon black, carbonnanotubes, inorganic fibers (carbon fibers, glass fibers, potassiumtitanate fibers and the like), organic fibers, metal powders (silver,copper, and the like) and waxes (polyethylene wax, polypropylene wax,and the like). Further as the filling agent, there can also be addedpiezoelectric particles (titanium oxide, barium titanate and the like),electroconductive particles (electroconductive carbon black,electroconductive titanium oxide, tin oxide and the like),thermoconductive particles (boron nitride and the like), organic fillers(silicone powder, polyethylene powder, polypropylene powder and thelike) and the like. In the case of using silica as the filling agent,the amount thereof to be added is, with respect to 100 parts by weightof the thermoplastic resin [solid content (nonvolatile content)],especially preferably in the range of 0.5 to 40 parts by weight. Furtherin the case of using clay such as mica as the filling agent, the amountthereof to be added is, with respect to 100 parts by weight of thethermoplastic resin [solid content (nonvolatile content)], especiallypreferably in the range of 0.3 to 10 parts by weight. Further in thecase of using a piezoelectric particle as the filling agent, the amountthereof to be added is, with respect to 100 parts by weight of thethermoplastic resin [solid content (nonvolatile content)], especiallypreferably in the range of 5 to 40 parts by weight. Further in the caseof using an electroconductive particle as the filling agent, the amountthereof to be added is, with respect to 100 parts by weight of thethermoplastic resin [solid content (nonvolatile content)], especiallypreferably in the range of 5 to 40 parts by weight. Further when apiezoelectric particle and an electroconductive particle are used incombination as the filling agent, the amount of charge to be generatedcan be regulated by the pressure. In this case, the ratio of thepiezoelectric particle to the electroconductive particle, for examplethe former/the latter (weight ratio) is preferably 10/90 to 90/10,preferably 20/80 to 80/20, and still more preferably 30/70 to 70/30.

The foamed sheet according to the present invention can be produced bysubjecting a resin composition containing the resin material (polymer)constituting the foamed body to expansion molding. As a foaming method(method of forming cells), there can be employed methods usually usedfor expansion molding, including physical methods and chemical methods.The physical methods generally involve dispersing a gas component suchas air or nitrogen in a polymer solution, and forming bubbles bymechanical mixing. The chemical methods are ones in which cells areformed by a gas generated by thermal decomposition of a foaming agentadded to a polymer base, to thereby obtain foamed bodies. From theviewpoint of environmental problems and the like, the physical methodsare preferable. Cells to be formed by the physical methods are opencells in many cases.

As the resin composition containing the resin material (polymer) to besubjected to expansion molding, there may be used a resin solution inwhich the resin material is dissolved in a solvent, but from theviewpoint of the foamability, an emulsion containing the resin materialis preferably used. Not less than two emulsions may be blended and usedas the emulsion.

It is preferable from the viewpoint of the film formability that thesolid content concentration of the emulsion is higher. The solid contentconcentration of the emulsion is preferably not less than 30% by weight,more preferably not less than 40% by weight, and still more preferablynot less than 50% by weight.

In the present invention, a method is preferable in which a foamed bodyis fabricated through a step (step A) of mechanically foaming anemulsion resin composition to thereby foam the emulsion resincomposition. A foaming apparatus is not especially limited, and examplesthereof include apparatuses such as a high-speed shearing system, avibration system, and a discharge system of a pressurized gas. Amongthese, from the viewpoint of the micronization of the cell diameter andthe fabrication of a large volume, a high-speed shearing system ispreferable.

Bubbles foamed by mechanical stirring are ones of gas entrapped in theemulsion. The gas is not especially limited as long as being inactive tothe emulsion, and includes air, nitrogen and carbon dioxide. Amongthese, from the viewpoint of the economical efficiency, air ispreferable.

By subjecting the emulsion resin composition foamed by the above methodto a step (step B) of coating a base material with the emulsion resincomposition followed by drying, the foamed sheet according to thepresent invention can be obtained. The base material is not especiallylimited, but examples thereof include release-treated plastic films(release-treated polyethylene terephthalate films and the like), plasticfilms (polyethylene terephthalate films and the like) andthermoconductive layers. In the case of coating by using athermoconductive layer as the base material, the close adhesion of afoamed body layer with the thermoconductive layer can be improved andthe efficiency of a drying step in the fabrication of the foamed bodylayer can also be improved.

As a coating method and a drying method in step B, usual methods can beemployed. It is preferable that step B comprises preliminary drying stepB1 of drying the bubble-containing emulsion resin composition applied onthe base material at not less than 50° C. and less than 125° C., andregular drying step B2 of thereafter further drying the resultant at notless than 125° C. and not more than 200° C.

By providing preliminary drying step B1 and regular drying step B2, thecoalescence of bubbles and the burst of bubbles due to a rapidtemperature rise can be prevented. Particularly in the foamed sheethaving a small thickness, since bubbles coalesce or burst in a rapidrise of the temperature, the significance of the provision ofpreliminary drying step B1 is large. The temperature in preliminarydrying step B1 is preferably not less than 50° C. and not more than 100°C. The time of preliminary drying step B1 is, for example, 0.5 min to 30min, and preferably 1 min to 15 min. Further the temperature in regulardrying step B2 is preferably not less than 130° C. and not more than180° C., and more preferably not less than 130° C. and not more than160° C. The time of regular drying step B2 is, for example, 0.5 min to30 min, and preferably 1 min to 15 min.

In the present invention, the average cell diameter, the maximum celldiameter and the minimum cell diameter of the foamed body can becontrolled by regulation of the kind and the amount of the surfactant,and regulation of the stirring rate and the stirring time in themechanical stirring.

Further the apparent density of the foamed body can be controlled byregulation of the amount of the gas component entrapped in the emulsionresin composition in the mechanical stirring.

Further the value of the compression set at 80° C. and the value of theimpact absorption change rate can be controlled, for example, byregulation of the degree of crosslinking and Tg of the resin material(polymer) constituting the foamed body. More specifically, for example,by regulation of the amount of the crosslinking agent to be added, andregulation of the proportion accounted for by the monomer having a Tg ofits homopolymer of not less than −10° C. in the whole monomer componentsforming the resin material (polymer), the value of the compression setat 80° C. and the value of the impact absorption change rate can becontrolled in predetermined ranges. By increasing the amount of thecrosslinking agent to be added, and increasing the proportion accountedfor by the monomer having a Tg of its homopolymer of not less than −10°C. in the whole monomer components forming the resin material (polymer),the value of the compression set at 80° C. and the value of the impactabsorption change rate can be made low.

The foamed sheet according to the present invention may have apressure-sensitive adhesive layer on one face or on both faces of thefoamed body. A pressure-sensitive adhesive constituting thepressure-sensitive adhesive layer is not especially limited, and may beany of an acrylic pressure-sensitive adhesive, a rubber-basedpressure-sensitive adhesive, a silicone-based pressure-sensitiveadhesive and the like. Further in the case of providing thepressure-sensitive adhesive layer, a release liner to protect thepressure-sensitive adhesive layer until its usage may be laminated onits face. Here, in the case where the foamed body constituting thefoamed sheet according to the present invention exhibits slighttackiness, members and the like can be fixed even without providing thepressure-sensitive adhesive layer.

The foamed sheet according to the present invention may be distributedto markets as a wound body (roll-like material) wound in a rolled form.

As described above, the foamed sheet according to the present invention,even if having a small thickness, is excellent in impact absorption.Further the foamed sheet is excellent in heat resistance, and even ifbeing subjected to compressions or impacts under high temperatures (forexample, about 80° C.), holds a force to recover its original shape(thickness). The foamed sheet according to the present invention has,for example, an 80° C. stress retention rate of not less than 68% asdefined by the following. Here, in conventional foamed sheets, the 80°C. stress retention rate is usually low, and the stress relaxes at 80°C. and the recovering force decays.

<The 80° C. Stress Retention Rate>

A test piece (foamed sheet) is held in an atmosphere of 80° C. for 30min; thereafter, by using a tensile tester, the test piece is set at 80°C. at an interchuck distance of 40 mm, stretched by 50% at a tensilerate of 500 mm/min, and thereafter held for 120 sec; a maximum load anda load after the 120 sec are measured, and the 80° C. stress retentionrate is determined by the following expression.

80° C. stress retention rate (%)=[a load(N) after the 120 sec/a maximumload(N)]×100

Thus, the foamed sheet according to the present invention, since even ifhaving a small thickness, being excellent in impact absorption andmoreover excellent in heat resistance, is high in installation(adhesion) reliability even at high temperatures; and for example, inelectric or electronic devices, the foamed sheet is useful as a member,particularly an impact absorption sheet, for electric or electronicdevices to be used when various types of members or components (forexample, optical members) are attached (installed) on predeterminedsites (for example, housings).

Examples of optical members attachable (installable) by utilizing thefoamed sheet according to the present invention include image displaymembers (particularly small-size image display members) installed onimage display apparatuses such as liquid crystal displays,electroluminescence displays and plasma displays, display members, suchas touch panels, installed on mobile communication apparatuses such asso-called “cellular phones,” “smartphones” and “personal digitalassistants,” cameras and lenses (particularly small-size cameras andlenses).

The electric or electronic device according to the present inventionuses the foamed sheet according to the present invention. Such anelectric or electronic device includes, for example, an electric orelectronic device having a display member, and having a structure inwhich the foamed sheet is interposed between a housing of the electricor electronic device and the display member. Examples of the electric orelectronic device include mobile communication apparatuses such asso-called “cellular phones,” “smartphones” and “personal digitalassistants.”

EXAMPLES

Hereinafter, the present invention will be described in more detail byway of Examples, but the present invention is not limited to theseExamples. Here, unless otherwise mentioned, “%” representing a contentmeans % by weight. Here, the numbers of parts (parts by weight) blendedare all values in terms of solid content (nonvolatile content).

Example 1

100 parts by weight of an acryl emulsion solution (the amount of thesolid content: 55%, an ethyl acrylate-butyl acrylate-acrylonitrilecopolymer (45:48:7 in weight ratio)), 1 part by weight of asilicone-based compound A (a dimethyl silicone oil, the number-averagemolecular weight Mn: 7.16×10³, the weight-average molecular weight Mw:1.71×10⁴, the amount of the solid content (nonvolatile content): 100%),3 parts by weight of a fatty acid ammonium-based surfactant (a waterdispersion of ammonium stearate, the amount of the solid content: 33%),2.0 parts by weight of an oxazoline-based crosslinking agent (“EpocrosWS-500,” manufactured by Nippon Shokubai Co., Ltd., the amount of thesolid content: 39%), 1 part by weight of a benzotriazole sodium salt(the solid content: 40%) (a rust preventive), and 0.8 parts by weight ofa polyacrylic acid-based thickener (an ethyl acrylate-acrylic acidcopolymer (acrylic acid: 20% by weight), the amount of the solidcontent: 28.7%) were stirred and mixed by a Disper (“Robomix,”manufactured by Primix Corp.) and thereby foamed. The foamed compositionwas applied on a release-treated PET (polyethylene terephthalate) film(the thickness: 38 μm, the trade name: “MRF#38,” manufactured byMitsubishi Plastics, Inc.), and dried at 70° C. for 4.5 min and 140° C.for 4.5 min to thereby obtain a foamed body (foamed sheet) of an opencell structure having a thickness of 100 μm, an apparent density of 0.34g/cm³, a cell porosity of 65.7%, a maximum cell diameter of 72.5 μm, aminimum cell diameter of 28.5 μm and an average cell diameter of 45 μm.

Example 2

100 parts by weight of an acryl emulsion solution (the amount of thesolid content: 55%, an ethyl acrylate-butyl acrylate-acrylonitrilecopolymer (45:48:7 in weight ratio)), 1 part by weight of asilicone-based compound A (a dimethyl silicone oil, the number-averagemolecular weight Mn: 7.16×10³, the weight-average molecular weight Mw:1.71×10⁴, the amount of the solid content (nonvolatile content): 100%),3 parts by weight of a fatty acid ammonium-based surfactant (a waterdispersion of ammonium stearate, the amount of the solid content: 33%),0.35 parts by weight of an oxazoline-based crosslinking agent (“EpocrosWS-500,” manufactured by Nippon Shokubai Co., Ltd., the amount of thesolid content: 39%), 1 part by weight of a benzotriazole sodium salt(the solid content: 40%) (a rust preventive), and 0.8 parts by weight ofa polyacrylic acid-based thickener (an ethyl acrylate-acrylic acidcopolymer (acrylic acid: 20% by weight), the amount of the solidcontent: 28.7%) were stirred and mixed by a Disper (“Robomix,”manufactured by Primix Corp.) and thereby foamed. The foamed compositionwas applied on a release-treated PET (polyethylene terephthalate) film(the thickness: 38 μm, the trade name: “MRF#38,” manufactured byMitsubishi Plastics, Inc.), and dried at 70° C. for 4.5 min and 140° C.for 4.5 min to thereby obtain a foamed body (foamed sheet) of an opencell structure having a thickness of 100 μm, an apparent density of 0.45g/cm³, a cell porosity of 54.5%, a maximum cell diameter of 87.5 μm, aminimum cell diameter of 48.5 μm and an average cell diameter of 65 μm.

Example 3

100 parts by weight of an acryl emulsion solution (the amount of thesolid content: 55%, an ethyl acrylate-butyl acrylate-acrylonitrilecopolymer (45:48:7 in weight ratio)), 1 part by weight of asilicone-based compound A (a dimethyl silicone oil, the number-averagemolecular weight Mn: 7.16×10³, the weight-average molecular weight Mw:1.71×10⁴, the amount of the solid content (nonvolatile content): 100%),3 parts by weight of a fatty acid ammonium-based surfactant (a waterdispersion of ammonium stearate, the amount of the solid content: 33%),0.35 parts by weight of an oxazoline-based crosslinking agent (“EpocrosWS-500,” manufactured by Nippon Shokubai Co., Ltd., the amount of thesolid content: 39%), 1 part by weight of a benzotriazole sodium salt(the solid content: 40%) (a rust preventive), and 0.8 parts by weight ofa polyacrylic acid-based thickener (an ethyl acrylate-acrylic acidcopolymer (acrylic acid: 20% by weight), the amount of the solidcontent: 28.7%) were stirred and mixed by a Disper (“Robomix,”manufactured by Primix Corp.) and thereby foamed. The foamed compositionwas applied on a release-treated PET (polyethylene terephthalate) film(the thickness: 38 μm, the trade name: “MRF#38,” manufactured byMitsubishi Plastics, Inc.), and dried at 70° C. for 4.5 min and 140° C.for 4.5 min to thereby obtain a foamed body (foamed sheet) of an opencell structure having a thickness of 120 μm, an apparent density of 0.26g/cm³, a cell porosity of 73.7%, a maximum cell diameter of 57.5 μm, aminimum cell diameter of 15.3 μm and an average cell diameter of 30 μm.

Example 4

100 parts by weight of an acryl emulsion solution (the amount of thesolid content: 55%, an ethyl acrylate-butyl acrylate-acrylonitrilecopolymer (45:48:7 in weight ratio)), 1 part by weight of asilicone-based compound A (a dimethyl silicone oil, the number-averagemolecular weight Mn: 7.16×10³, the weight-average molecular weight Mw:1.71×10⁴, the amount of the solid content (nonvolatile content): 100%),3 parts by weight of a fatty acid ammonium-based surfactant (a waterdispersion of ammonium stearate, the amount of the solid content: 33%),1 part by weight of a benzotriazole sodium salt (the solid content: 40%)(a rust preventive), and 0.8 parts by weight of a polyacrylic acid-basedthickener (an ethyl acrylate-acrylic acid copolymer (acrylic acid: 20%by weight), the amount of the solid content: 28.7%) were stirred andmixed by a Disper (“Robomix,” manufactured by Primix Corp.) and therebyfoamed. The foamed composition was applied on a release-treated PET(polyethylene terephthalate) film (the thickness: 38 μm, the trade name:“MRF#38,” manufactured by Mitsubishi Plastics, Inc.), and dried at 70°C. for 4.5 min and 140° C. for 4.5 min to thereby obtain a foamed body(foamed sheet) of an open cell structure having a thickness of 130 μm,an apparent density of 0.37 g/cm³, a cell porosity of 62.6%, a maximumcell diameter of 82.5 μm, a minimum cell diameter of 43.5 μm and anaverage cell diameter of 60 μm.

Comparative Example 1

45 parts by weight of a polypropylene [the melt flow rate (MFR): 0.35g/10 min], 55 parts by weight of a mixture (MFR (230° C.): 6 g/10 min,JIS A-hardness: 79°, 30 parts by mass of a softening agent was blendedin 100 parts by mass of a polyolefinic elastomer) of the polyolefinicelastomer and the softening agent (paraffinic extender oil), 10 parts byweight of magnesium hydroxide, 10 parts by weight of a carbon (the tradename: “Asahi #35,” manufactured by Asahi Carbon Co., Ltd.), 1 part byweight of stearic monoglyceride, and 1.5 parts by weight of a fatty acidamide (lauric acid bisamide) were kneaded in a twin-screw kneader,manufactured by The Japan Steel Works, Ltd. (JSW), at a temperature of200° C., then extruded in a strand form, and water cooled and thenformed into a pellet form. The pellet was charged in a single-screwextruder, manufactured by The Japan Steel Works, Ltd.; and a carbondioxide gas was injected in the atmosphere of 220° C. at a pressure of13 (after the injection, 12) MPa. The carbon dioxide gas was injected ina proportion of 5.6% by weight with respect to the total amount of thepellet. After the carbon dioxide gas was fully saturated, the resultantwas cooled to a temperature suitable for being foamed, and thereafterextruded in a cylindrical form through a die; the resultant was passedthrough between a mandrel to cool the inner side face of the foamed bodyand an air ring for cooling the foamed body to cool the outside face ofthe cylindrical foamed body extruded from the ring die of the extruder;and a part of the diameter was cut and the cylindrical foamed body wasunfolded into a sheet form to thereby obtain a long-size foamed bodyoriginal sheet. In the long-size foamed body original sheet, the averagecell diameter was 55 μm, and the apparent density was 0.041 g/cm³.

The long-size foamed body original sheet was cut (subjected to aslitting work) into a predetermined width; and by using a continuousslicing apparatus (slicing line), high-density layers on faces wereseparated away one by one to thereby obtain a resin foamed body.

By passing the resin foamed body through in the above continuoustreatment apparatus in which the temperature of the induction heatingroll was set at 160° C. and the gap was set at 0.20 mm, one face thereofwas subjected to a melting treatment by the heat; and the resultant wassubjected to a slitting work, and thereafter taken up to thereby obtaina wound body. Here, the taking-up speed was made to be 20 m/min.

Then, by rewinding the wound body, and passing the rewound body throughin the above continuous treatment apparatus in which the temperature ofthe induction heating roll was set at 160° C. and the gap was set at0.10 mm, a face thereof (untreated face) not having been subjected to amelting treatment was subjected to a melting treatment by the heat; andthe resultant was subjected to a slitting work, and thereafter taken upto thereby obtain a foamed body (foamed sheet) whose both faces had beensubjected to the heat melting treatment and which had an open cellstructure having a thickness of 100 μm, an apparent density of 0.12g/cm³, a cell porosity of 88%, a maximum cell diameter of 90 μm, aminimum cell diameter of 30 μm and an average cell diameter of 60 μm.

<Evaluations>

The foamed bodies (foamed sheets) obtained in the Examples and theComparative Example were evaluated for the following. The results areshown in Table 1 and Table 2. In Table 1, there is shown the number ofparts (parts by weight) [in terms of solid content (nonvolatilecontent)] of each component blended in each Example and ComparativeExample. “Em” indicates an emulsion.

(The Average Cell Diameter)

The average cell diameter (μm) was determined by taking and imageanalyzing an enlarged image of a foamed body cross-section by alow-vacuum scanning electron microscope (“S-3400N type scanning electronmicroscope,” manufactured by Hitachi High-Tech Science Systems Corp.).Here, the number of cells analyzed was about 10 to 20. Similarly, theminimum cell diameter (μm) and the maximum cell diameter (μm) of thefoamed sheet were determined.

(The Apparent Density)

A foamed body (foamed sheet) is punched out with a punching knife of 100mm×100 mm, and the size of the punched-out sample is measured. Furtherthe thickness is measured by a 1/100 dial gage having a diameter (φ) ofits measuring terminal of 20 mm. The volume of the foamed body wascalculated from these values.

Then, the weight of the foamed body is measured by an even balance whoseminimum division is not less than 0.01 g. The apparent density (g/cm³)of the foamed body was calculated from these values.

(The Dynamic Viscoelasticity)

A temperature-dispersion test was carried out at an angular frequency of1 rad/sec in a film tensile measurement mode of a viscoelasticitymeasuring apparatus (“ARES2KFRTN1-FCO,” manufactured by TA InstrumentsJapan Inc.). There was measured the temperature (° C.) and intensity(maximum value) of the peak top of the loss tangent (tan δ), which was aratio of the loss elastic modulus E″ to the storage elastic modulus E′at this time.

In the column of “tan δ Temperature” of Table 2, the temperatures (° C.)of peak tops of loss tangents (tan δ) of foamed bodies are indicated;and in the column of “tan δ Maximum Value,” intensities (maximum values)of the peak tops are indicated.

(The Compression Set Test)

The foamed sheets (sample size: 30 mm×30 mm) obtained in the Examplesand the Comparative Example were used as test pieces. By using the testpiece, the compression set test was carried out at 80° C. (according tothe provision of JIS K6262). More specifically, the test piece wascompressed (until the thickness of the compressed test piece became athickness of 40% of its original thickness) in an atmosphere of 80° C.,held in this state for 24 hours, thereafter released from the compressedstate, and left as it was at 23° C. for 30 min; and the thickness of thetest piece was measured at 23° C. Then, the compression set (%) at 80°C. was determined by the following expression.

CS={(t0−t1)/(t0−t2)}×100

CS: a compression set (%)

t0: an original thickness (mm) of a test piece

t1: a thickness (mm) of the test piece at 30 min after the test piece isremoved from a compression apparatus

t2: a thickness (mm) of the test piece in the state of being under acompressive strain

(The Impact Absorption Change Rate)

For the foamed sheets (sample size: 20 mm×20 mm) (test pieces A)obtained in the Examples and the Comparative Example, by using theabove-mentioned pendulum impact tester (impact testing apparatus) (seeFIG. 1 and FIG. 2), an impact absorption test was carried out under theconditions of 23° C., a weight of an impactor of 28 g, and a swing-upangle of 40°. The impact absorption rate acquired at this time isdefined as an initial impact absorption rate a.

Then, the test piece A was stored at 80° C. for 72 hours in the state ofbeing compressed by 60% with respect to the initial thickness of thetest piece A, and thereafter, the compression state was released; andthereafter, as in the above, an impact absorption test was carried outunder the conditions of 23° C., a weight of an impactor of 28 g, and aswing-up angle of 40° after a lapse of 24 hours at 23° C. The impactabsorption rate acquired at this time is defined as an impact absorptionrate b after the high-temperature compression.

Then, the impact absorption change rate (%) was determined by thefollowing expression.

Impact absorption change rate (%)={(the impact absorption rate b afterhigh-temperature compression−the initial impact absorption rate a)/theinitial impact absorption rate a}×100

Here, the impact absorption rate is a value defined by the followingexpression.

Impact absorption rate (%)={(F ₀ −F ₁)/F ₀}×100

wherein F₀ is an impact force when the impactor is made to collide witha support plate alone; and F₁ is an impact force when the impactor ismade to collide with the support plate of a structural body composed ofthe support plate and the test piece A.

(The 80° C. Stress Retention Rate)

The foamed sheets [the shape and size of the samples: dumbbell No. 1(see JIS K6251)] obtained in the Examples and the Comparative Examplewere each held in an atmosphere of 80° C. for 30 min, thereafter, byusing a tensile tester, set at 80° C. at an interchuck distance of 40mm, stretched by 50% at a tensile speed of 500 mm/min, and thereafterheld for 120 sec; and the maximum load and the load after the 120 secwere measured and the 80° C. stress retention rate was determined by thefollowing expression.

80° C. stress retention rate (%)=[a load (N) after 120 sec/a maximumload (N)]×100

TABLE 1 Silicone- Surfactant Em based (Foaming Thickener Rust theCompound Agent) Crosslinking the Preventive number the number the numberAgent number of the number of parts of parts of parts the number ofparts of parts blended blended blended parts blended blended blendedExample 1 100 1 3 2   0.8 1 2 100 1 3 0.35 0.8 1 3 100 1 3 0.35 0.8 1 4100 1 3 0.35 0.8 1 Comparative 1 — — — — — — Example

TABLE 2 Impact Absorption Initial Rate b after Impact 80° C. Averagetanδ Impact High- Absorption Stress Cell Apparent tanδ MaximumAbsorption Temperature Change Compression Retention Thickness DiameterDensity Temperature Value Rate a Compression Rate Set Rate (μm) (μm)(g/cm³) (° C.) (—) (%) (%) (%) (%) (%) Example 1 100 45 0.34 −3 0.37 3332 −3 0 74 2 100 65 0.45 −3 0.38 30 30 0 0 76 3 120 30 0.26 −2 0.42 35.431.2 −12 14 73 4 130 60 0.37 −3 0.37 34.8 31.3 −10 13 73 Comparative 1100 60 0.12 0 0.21 26 13 −50 92 64 Example

INDUSTRIAL APPLICABILITY

The foamed sheet according to the present invention, since even ifhaving a small thickness, being excellent in impact absorption andmoreover excellent in heat resistance, is high in installation(adhesion) reliability even at high temperatures; and for example, inelectric or electronic devices, the foamed sheet is useful as a member,particularly an impact absorption sheet, for electric or electronicdevices to be used when various types of members or components (forexample, optical members) are attached (installed) on predeterminedsites (for example, housings). Examples of optical members attachable(installable) by utilizing the foamed sheet according to the presentinvention include image display members (particularly small-size imagedisplay members) installed on image display apparatuses such as liquidcrystal displays, electroluminescence displays and plasma displays,display members, such as touch panels, installed on mobile communicationapparatuses such as so-called “cellular phones,” “smartphones” and“personal digital assistants,” cameras and lenses (particularlysmall-size cameras and lenses). The electric or electronic deviceaccording to the present invention uses the foamed sheet according tothe present invention. Such an electric or electronic device includes,for example, an electric or electronic device having a display member,and having a structure in which the foamed sheet is interposed between ahousing of the electric or electronic device and the display member.Examples of the electric or electronic device include mobilecommunication apparatuses such as so-called “cellular phones,”“smartphones” and “personal digital assistants.”

REFERENCE SIGNS LIST

-   -   1 PENDULUM IMPACT TESTER (IMPACT TESTING APPARATUS)    -   2 TEST PIECE (FOAMED SHEET)    -   3 HOLDING MEMBER    -   4 IMPACT APPLYING MEMBER    -   5 PRESSURE SENSOR    -   11 FIXING JIG    -   12 PRESSING JIG    -   16 PRESSURE ADJUSTING MEANS    -   20 SUPPORT COLUMN    -   21 ARM    -   22 ONE END OF SUPPORT ROD (SHAFT)    -   23 SUPPORT ROD (SHAFT)    -   24 IMPACTOR    -   25 ELECTROMAGNET    -   28 SUPPORT PLATE    -   a SWING-UP ANGLE

1. A foamed sheet, comprising a foamed body having an average celldiameter of 10 to 200 μm, and having a compression set at 80° C. of notmore than 80% and an impact absorption change rate of not more than ±20%as defined by the following:an impact absorption change rate (%)={(an impact absorption rate b afterhigh-temperature compression−an initial impact absorption rate a)/theinitial impact absorption rate a}×100, wherein the initial impactabsorption rate a: an impact absorption rate (%) of a test piece A; theimpact absorption rate b (%) after high-temperature compression: animpact absorption rate (%) acquired by storing the test piece A at 80°C. for 72 hours in the state of being compressed by 60% with respect toan initial thickness of the test piece A, thereafter releasing thecompression state, and thereafter conducting measurement after a lapseof 24 hours at 23° C.; and the impact absorption rate: a value definedby the following expression in an impact absorption test (a weight of animpactor: 28 g, a swing-up angle: 40°) (23° C.) using a pendulum impacttester:an impact absorption rate (%)={(F ₀ −F ₁)/F ₀}×100 wherein F₀ is animpact force when the impactor is made to collide with a support platealone; and F₁ is an impact force when the impactor is made to collidewith the support plate of a structural body composed of the supportplate and the test piece A.
 2. The foamed sheet according to claim 1,wherein the foamed sheet has a thickness of 30 to 1,000 μm; and thefoamed body has an apparent density of 0.2 to 0.7 g/cm³.
 3. The foamedsheet according to claim 1, wherein the foamed body has a peak top of aloss tangent (tan δ) in the range of not less than −30° C. and not morethan 30° C., the loss tangent (tan δ) being a ratio of a loss elasticmodulus to a storage elastic modulus at an angular frequency of 1rad/sec in a dynamic viscoelasticity measurement.
 4. The foamed sheetaccording to claim 1, wherein the foamed body is formed of at least oneresin material selected from the group consisting of acrylic polymers,rubbers, urethanic polymers and ethylene-vinyl acetate copolymers. 5.The foamed sheet according to claim 1, wherein the foamed body is formedthrough step A of mechanically foaming an emulsion resin composition. 6.The foamed sheet according to claim 5, wherein the foamed body is formedfurther through step B of coating a base material with the mechanicallyfoamed emulsion resin composition followed by drying.
 7. The foamedsheet according to claim 6, wherein step B comprises preliminary dryingstep B1 of drying the bubble-containing emulsion resin compositionapplied on the base material at not less than 50° C. and less than 125°C., and regular drying step B2 of thereafter further drying theresultant at not less than 125° C. and not more than 200° C.
 8. Thefoamed sheet according to claim 1, wherein the foamed sheet has acompression set at 80° C. of not more than 50%.
 9. The foamed sheetaccording to claim 8, wherein the foamed sheet has the compression setat 80° C. of not more than 25%.
 10. The foamed sheet according to claim2, wherein the foamed sheet has a thickness of 40 to 500 μm.
 11. Thefoamed sheet according to claim 10, wherein the foamed sheet has athickness of 50 to 300 μm.
 12. The foamed sheet according to claim 2,wherein the foamed body has an apparent density of 0.21 to 0.6 g/cm³.13. The foamed sheet according to claim 12, wherein the foamed body hasan apparent density of 0.22 to 0.5 g/cm³.
 14. The foamed sheet accordingto claim 1, wherein the foamed sheet has a pressure-sensitive adhesivelayer on one face or both faces of the foamed body.
 15. The foamed sheetaccording to claim 1, being used as an impact absorption sheet for anelectric or electronic device.
 16. An electric or electronic device,using a foamed sheet according to claim
 1. 17. An electric or electronicdevice comprising a display member, a housing, and the foamed sheetaccording to claim 1 between the housing of the electric or electronicdevice and the display member.